The prior art is replete with processes for the production of essentially pure nitrogen gas. Many of such processes are highly efficient for producing relatively small or medium quantities of nitrogen. However, at the relatively large quantities per day of nitrogen production which would be required by enhanced petroleum recovery operations utilizing nitrogen as a pressurizing or pressure maintaining phase, these known prior art processes have relatively reduced process efficiencies in comparison to the present invention.
In the production of nitrogen, the prior art has utilized various forms of apparatus including double columns and single columns. In addition, various flow streams to and from these columns have been utilized in attempting to provide the most efficient production of nitrogen.
U.S. Pat. No. 3,492,828 discloses several cycles including a cycle for the production of liquid nitrogen and liquid oxygen, FIG. 3. A minor amount of nitrogen gas can optionally be recovered. However, the nitrogen recycle includes partial compression, partial expansion and a column in which only one reflux stream is provided. Crude oxygen expansion is not utilized for the provision of refrigeration.
Exemplary of additional prior art is U.S. Pat. No. 4,222,756 to Robert M. Thorogood which discloses a two column nitrogen separation process and apparatus. The patented process is designed to produce large tonnages of nitrogen gas. In this patented process, the reflux and the reboil to the double column is supplied directly by the feed air stream and by directing nitrogen from the high pressure column to the low pressure column. Additional nitrogen generators of the prior art are set forth in the above-identified patent, all of which have a greater power requirement than the present invention at large tonnage capacity, from 15 MMSCFD up to 200 MMSCFD, or more.
The present invention provides a process and apparatus for the production of relatively large quantities of nitrogen, which has an efficiency over the prior art, such as Thorogood and the cycle disclosed in FIG. 4 of Thorogood, in the range of 1.5 to 7% savings in energy consumption. In addition, the prior art nitrogen generators have generally recovered around 50% of the overall nitrogen which is processed, whereas the present invention can recover preferably anywhere from 70 to 87% of the nitrogen processed, depending on the recyle stream flow rate. This recovery is achieved in part from the additional boil-up vapor flow and additional reflux flow provided by the recycle circuit of the present invention.
It is an object of the present invention to provide a nitrogen generator utilizing a single column.
It is another object of the present invention to provide a large quantity nitrogen generator which utilizes a nitrogen recycle to provide reboil and to augment the reflux to the single column of the generator unit.
It is yet another object of the present invention to provide an efficient recovery process and apparatus for large tonnages of nitrogen wherein process efficiencies are improved from 1.5% to 7% in power consumption over the prior art processes for nitrogen production in plants preferably of 15 MMSCFD to 200 MMSCFD.